A turbine includes a turbine rotor, and a plurality of moving blades supported on the turbine rotor. By rotating the moving blades by means of a working fluid, the turbine rotor is driven. In recent years, attempts to elevate a temperature of the working fluid have been made in order to improve a turbine efficiency. With the working fluid having a higher temperature, some components are required to be made of a heat-resistant alloy.
In particular, in a part of the turbine rotor supporting the moving blades, i.e., a rotor wheel where a large stress is generated, lowering of strength caused by a high temperature is required to be restrained, as well as the heat resistance by the heat-resistant alloy is required to be ensured. There is proposed a technique for cooling the rotor wheel of the turbine rotor so as to restrain lowering of the strength of the rotor wheel.
When cooling the rotor wheel by a cooling fluid, a plurality of cooling-fluid introducing passages through which a cooling fluid flows are typically formed in the turbine rotor, and the rotor wheel is cooled by a cooling fluid having passed through the cooling-fluid introducing passages. The cooling fluid having cooled the rotor wheel merges into a working fluid for driving the moving blades. Thus, the higher a flow rate of the cooling fluid is, the greater the cooling fluid exerts an influence on a temperature of the working fluid, which lowers the turbine efficiency.
Thus, the flow rate of the cooling fluid is required to be made minimum, depending on a flow rate of the working fluid and an output taken out from the turbine rotor. However, after the cooling-fluid introducing passages have been formed in the turbine rotor, to change the flow rate of the cooling fluid is required a lot of effort and time. Namely, it is required that the turbine is disassembled to take out the turbine rotor, and then that the cooling-fluid introducing passages formed in the turbine rotor are refabricated.